The photometabolism of bilirubin is to be studied by broad band fluorescent radiation of jaundiced Gunn rats fitted with a bile duct cannula. The photoproducts of bilirubin are to be isolated from the bile by thin layer and high pressure liquid chromatography and identified as either photo-isomers, photo-oxygenations and/or photo-adducts of bilirubin. Similar studies are to be performed using as the light source monochromatic light from an argon tunable lasar. The latter will permit the determination of the in-vivo action spectrum of bilirubin. The cholestasis and diarrhea associated with the clinical use of phototherapy for neonatal hyperbilirubinemia is to be quantified by serial determination of serum concentrations of bile acids and conjugated bilirubin and intestinal transit times. Amelioration and prevention of the cholestasis and diarrhea is to be studied by blocking the enterohepatic circulation of bilirubin during phototherapy by the oral administration of USP agar. Stool samples are to be analyzed for total bile pigment excretions and quantitatively examined for the presence of photoproducts of bilirubin. The presence of and relative magnitude of amino acid conjugates of bilirubin are to be quantitated in the stool bile pigments of premature infants and in the bile of patients with deficiencies in their hepatic UDP-glucuronyl transferase. The intrahepatic metabolic pathway of the formation of such conjugates is to be studied in-vitro in liver isolates of experimental animals.